At some point during the evolution of the Black Widow it developed a venom that contains a chemical that is meant only to kill vertebrates. This is odd since most of the animals that black widows hunt are not vertebrates. At the same time most of the chemicals in their venom kill only other insects. So why did black widows develop a venom that could harm animals with a spine, which of course, includes us humans? Scientists do not know.
The brown recluse is another example of a spider with an even more terrifying venom. A chemical in the brown recluse's venom specifically eats away at flesh, and so do its ancestors. The assassin spider of africa shares a common ancestor with the brown recluse but neither of these spiders hunt anything with flesh. Brown recluse spiders and assassin spiders avoid humans whenever possible. These spiders opt to run away rather than stick around and attack a human that is clearly at the advantage. Fatal spider bites may be rare, but it would be foolish to not be on the lookout whenever you are outdoors.
What spider terrifies you the most and why?